Metal-free few-layer phosphorous nanomaterial: method for its preparation and use thereof

ABSTRACT

A method for preparing a metal-free few-layer phosphorous nanomaterial. The method comprises an ice-assisted exfoliation process (or solvent ice-assisted exfoliation process). The method allows for the preparation of a few-layer phosphorous nanomaterial with improved yield and reduced duration and exfoliation power. The few-layer phosphorous nanomaterial is used in the preparation of a photocatalyst. The photocatalyst exhibits a long-term stability, high photocatalytic H2 evolution efficiency from water, and good stability under visible light irradiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 62/685,371, filed on Jun. 15, 2018, the content of which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates generally to few-layer phosphorousnanomaterials. More specifically, the present invention relates to ametal-free few-layer black phosphorous (BP) nanomaterial. The method forits preparation comprises an ice-assisted exfoliation process. The BPnanomaterial according to the invention may be used, among others, inthe development of photocatalysts.

BACKGROUND OF THE INVENTION

Solar water splitting for H₂ evolution has shown great potential as agreen technology in solving energy crisis [1]. Taking economic andenvironmental factors into consideration, the development of efficient,low-cost, stable and nontoxic photocatalyst is highly desired for awidespread implementation of solar fuel technology. In this regard,visible-light-responsive graphitic carbon nitride (g-C₃N₄), atwo-dimensional (2D) metal-free photocatalyst, has been extensivelyexplored in photocatalysis. Though g-C₃N₄ was discovered to be feasiblefor photocatalytic water splitting, achieving an acceptable efficiencyin H₂ evolution still relies largely on the loading of noble metalco-catalysts. This is necessary because of the high recombination rateof the charge carriers in g-C₃N₄ [2]. Furthermore, the relatively widebandgap (2.7 eV) confines its light response mainly into theultraviolent (UV) range and only slightly into a small portion of thevisible light range (λ<460 nm) [3]. To solve these problems, numerousstrategies have been developed, mainly including morphology tuning,doping with metal/non-metal ions, and heterojunction creation [4].However, quite limited progresses have been achieved thus far. Aiming toenhance the harvesting of solar light efficiently and economically, thedevelopment of g-C₃N₄-based metal-free photocatalysts with a broaderphoto-response range is of great significance.

Black phosphorus (BP), a layered material that consists of corrugatedatomic planes with strong intra-layer chemical bonding and weakinterlayer Van der Waals interactions, has attracted the interest ofmaterial scientists. Since the successful preparation of 2D BP withatom-thick layer in early 2014, it has provoked a surge of research withits enticing electrical and optical properties [5]. Differentiating frompreviously reported 2D nanomaterial such as graphene, BP possesses atunable thickness-dependent bandgap that spans from about 0.3 eV (bulk)to about 2.0 eV (monolayer) in addition to sufficiently high carriermobility and photo-electronic response [5b-d, 5f, 5g, 5i]. Thesefavorable properties render BP, particularly few-layer BP nanosheets (10nm in thickness), a good candidate for diverse applications intransistor and photodetector devices, solar cells, bio-imaging andphototherapy [5i, 6]. Notably, BP has demonstrated its great potentialas a broadband photocatalyst for the harvesting of solar energy due toits narrow and direct bandgap [7].

However, certain inherent problems existing in the typical, exfoliatedBP nanosheets bring practical challenges for its actual application. Forexample, BP is very reactive to moisture and ambient oxygen, and can beeasily oxidized due to the exposed lone pairs at its surface [5f, 6e,7e, 8]. The roughening caused by the exfoliation can further acceleratethe surface oxidation, which may proceed exponentially during the firsthour after exfoliation [8b]. As a consequence, the semiconductingproperties of BP deteriorate rapidly, reflected from significantlyincreased contact resistance and reduced carrier mobility [8a, 8b, 8e].It is thus importance to develop effective strategies to retard oreliminate the degradation of BP.

Recently, several approaches were developed to protect BP from oxidationwith various levels of success [5i, 9]. Among these approaches, thenon-covalent surface coverage of BP with other inert 2D materials, suchas poly (methyl methacrylate), graphene or hexagonal boron nitride, wasproposed [8e, 9b].

For the preparation of few-layer BP nanosheets, the mechanical andliquid exfoliation from bulk BP is known in the art [8a, 8c, 10]. As BPpossesses stronger interlayer interactions compared to graphene or other2D materials, the exfoliation by ultrasonication would be difficult andwould require a long processing time (>15 hours), or would require asonicator with high power [8a, 8c, 10]. The yield obtained for thepreparation of few-layer BP nanosheets is still low [8a, 10c]. As theP-P bond is weaker than the C—C bond, such long duration or high powerof sonication are known to generate nanosheets with reduced lateral sizeand structural defects [8a, 11]. In addition to the instability, suchstructural defects also restrict the practical applications of BPobtained by these methods.

There is a need for few-layer phosphorous nanomaterials that are stable,that have structures free of defects, and that are environment-friendly.There is a need for efficient methods for the preparation of suchfew-layer phosphorous nanomaterials.

SUMMARY OF THE INVENTION

The inventors have designed and performed a method for preparing ametal-free few-layer phosphorous nanomaterial. The method comprises anice-assisted exfoliation process (or solvent ice-assisted exfoliationprocess). The method according to the invention is novel, and allows forthe preparation of a few-layer phosphorous nanomaterial with improvedyield and reduced duration and exfoliation power.

In embodiments of the invention, the inventors have designed andperformed a method for preparing a metal-free few-layer blackphosphorous (BP) nanomaterial. In these embodiments, the ice-assistedexfoliation process involves use of a solvent. Preferably, the solventis an organic solvent, for example N-methyl-2-pyrrolidone (NMP).

In other embodiments of the invention, a photocatalyst is prepared. Inthese embodiments, the few-layer BP nanomaterial and graphitic carbonnitride (g-C₃N₄) are integrated into a single, 2D-on-2D architecture(BP/g-C₃N₄). The thus-obtained metal-free BP/g-C₃N₄ photocatalystexhibits a long-term stability, high photocatalytic H₂ evolutionefficiency from water, and good stability under visible lightirradiation.

The invention thus provides the following according to aspects thereof:

-   -   (1) Method for preparing a few-layer phosphorous nanomaterial        from a bulk layer-structured phosphorous material, comprising an        ice-assisted exfoliation process or solvent ice-assisted        exfoliation process.    -   (2) Method for preparing a few-layer phosphorous nanomaterial        from a bulk layer-structured phosphorous material, comprising a        combination of the following steps: grinding, dispersion in a        solvent, freezing, melting, separation, purification.    -   (3) Method for preparing a few-layer phosphorous nanomaterial,        comprising: (a) providing a bulk layer-structured phosphorous        material; (b) grinding the bulk phosphorous material; (c)        dispersing the grinded material into a first solvent to obtain a        first dispersion; (d) freezing the first dispersion for a period        of time, preferably using liquid nitrogen; (e) melting the        frozen dispersion, preferably by sonication for a period of time        to obtain a second dispersion; and (f) submitting the second        dispersion to a separation step, preferably involving        centrifugation for a period of time, to obtain the nanomaterial.    -   (4) Method according to (3) above, further comprising a        purification step; preferably the purification step        comprises: (g) washing the nanomaterial using a second solvent,        optionally repeating step (g) a number of time, preferably 2-6        times, or 3 times, or 4 times; and (h) dispersing the        nanomaterial into a third solvent, wherein the second and third        solvents are the same or different.    -   (5) Method according to (3) or (4) above, wherein steps (d)        and (e) are repeated a number of time, preferably 2 to 6 times,        or 3 times or 4 times.    -   (6) Method according to (3) above, wherein the freezing time        period at step (d) is about 3-15 minutes, or about 4-14 minutes,        or about 5-13 minutes, or about 5-12 minutes, or about 5-11        minutes, or about 5-10 minutes, or about 6-8 minutes.    -   (7) Method according to (3) above, wherein the sonication time        period at step (e) is about 5-15 minutes, or about 6-14 minutes,        or about 7-13 minutes, or about minutes 8-12 minutes, or about        9-11 minutes, or about 10 minutes.    -   (8) Method according to (3) above, wherein the centrifugation at        step (f) is performed at 7000 rpm and the time period is about        10-20 minutes, or about 12-18 minutes, or about 14-16 minutes,        or about 15 minutes.    -   (9) Method according to any one of (1) to (8) above, wherein the        bulk layered structure phosphorous material is black phosphorous        (BP), red phosphorous (RP), violet phosphorous (VP).    -   (10) Method according to any one of (1) to (9) above, wherein        the bulk layer-structured phosphorous material is a black        phosphorous (BP) material, and the few-layer phosphorous        nanomaterial is a few-layer black phosphorous (BP) nanomaterial.    -   (11) Method according to (1) above, wherein the solvent is an        organic solvent; preferably the organic solvent is selected from        the group consisting of N-methyl-2-pyrrolidone (NMP), alcohols        such as methanol, ethanol and isopropanol (IPA), diethyl ether,        chloroform, tetrahydrofuran, cyclohexane, toluene,        dimethylformamide, and combinations thereof; more preferably the        solvent is N-methyl-2-pyrrolidone (NMP).    -   (12) Method according to (3) or (4) above, wherein: the first        solvent is selected from the group consisting of        N-methyl-2-pyrrolidone (NMP), alcohols such as methanol, ethanol        and isopropanol (IPA), diethyl ether, chloroform,        tetrahydrofuran, cyclohexane, toluene, dimethylformamide, and        combinations thereof; preferably the first solvent is        N-methyl-2-pyrrolidone (NMP); the second solvent is selected        from the group consisting of isopropanol (IPA), other alcohols        such as methanol and ethanol; diethyl ether, chloroform,        tetrahydrofuran, cyclohexane, toluene, dimethylformamide, and        combinations thereof; preferably the second solvent is        isopropanol (IPA); and the third solvent is selected from the        group consisting of isopropanol (IPA), other alcohols such as        methanol and ethanol; diethyl ether, chloroform,        tetrahydrofuran, cyclohexane, toluene, dimethylformamide,        N-methyl-2-pyrrolidone (NMP), and combinations thereof;        preferably the second solvent is isopropanol (IPA).    -   (13) Method according to any one (1) to (12) above, wherein        substantially no oxidation occurs.    -   (14) Method according to any one (1) to (12) above, wherein the        few-layer phosphorous nanomaterial is metal-free.    -   (15) A few-layer phosphorous nanomaterial obtained by the method        as defined in any one of (1) to (14) above.    -   (16) A few-layer black phosphorous (BP) nanomaterial obtained by        the method as defined in any one of (1) to (14) above.    -   (17) A few-layer phosphorous nanomaterial as defined in (15)        or (16) above, having 4 to 10 layers, or 5 to 9 layers, or 6 to        8 layers, or 7 layers, or 6 layers.    -   (18) A few-layer phosphorous nanomaterial as defined in any one        of (15) to (17) above, having a thickness which is less than        about 12 nm, or less than about 10 nm; or which is about 9 nm,        or about 8 nm, or about 7 nm, or about 6 nm, or about 5 nm.    -   (19) Use of a few-layer phosphorous nanomaterial as defined in        any one of (15) to (18) above, in the development of        photocatalysts, transistor devices, photodetector devices, solar        cells, or in bio-imaging, or in phototherapy.    -   (20) A method for preparing a photocatalyst, comprising coupling        the few-layer phosphorous nanomaterial as defined in any one        of (15) to (18) above, with a 2D material; preferably the 2D        material is selected from the group consisting of poly (methyl        methacrylate), graphene or hexagonal boron nitride which may be        nitrogen-doped, molybdenum disulfide, a carbon nitride        nanomaterial; more preferably the 2D material is graphitic        carbon nitride (g-C₃N₄).    -   (21) A method for preparing a photocatalyst, comprising coupling        the few-layer black phosphorous (BP) nanomaterial as defined        in (20) above, with graphitic carbon nitride (g-C₃N₄).    -   (22) Use of the few-layer phosphorous nanomaterial as defined in        any one of (15) to (18) above, in the preparation of a        photocatalyst.    -   (23) Use of the few-layer black phosphorous (BP) nanomaterial as        defined in (16) above, in the preparation of a photocatalyst.    -   (24) A photocatalyst obtained by the method as defined in (20)        or (21) above.    -   (25) A photocatalyst obtained by the method as defined in (21)        above, which is few-layer black phosphorous nanomaterial/g-C₃N₄.    -   (26) Use of the photocatalyst as defined in (24) or (25) above,        for water splitting (H₂ evolution).

Other objects, advantages and features of the present invention willbecome more apparent upon reading of the following non-restrictivedescription of specific embodiments thereof, given by way of exampleonly with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

In the appended drawings:

FIG. 1: (a) Schematic illustration of the preparation of BP nanosheetswith ice-assisted exfoliation method. (b) TEM image of BP nanosheets and(c) EDX spectrum of (b). (d) Tapping mode AFM topographical image offew-layer BP nanosheets. Scale bars in b) and (d) are 500 nm. (e) Theheight profiles of BP nanosheets along the blue Line 1 and green Line 2in (d). (f) Statistical thickness distribution calculated from theheight profiles of 150 BP nanosheets in AFM images.

FIG. 2: Photographs of BP nanosheets in isopropanol (IPA) (a) at thefirst day, (b) after four weeks, (c) after adding g-C₃N₄, and (d) afterthe incubation at room temperature for 30 minutes. (e) The zetapotentials of BP and g-C₃N₄ nanosheets in IPA.

FIG. 3: P2p XPS spectra of BP and BP/g-C₃N₄ samples after watersplitting under visible light irradiation for 24 hours.

FIG. 4: Representative TEM images of (a) g-C₃N₄ and (b-d) BP/g-C₃N₄ withdifferent magnifications. (e) High-angle annular dark field (HAADF)scanning TEM (STEM) image of (d), (f-i) STEM-EDX mapping of C, N, P, andthe overlay of all the elements of the selected area in (e). (j) HRTEMimage of BP/g-C₃N₄, and (k) EDX spectrum of (j). Scale bars: (a) and(c-i), 250 nm; (b), 1 μm; (j), 5 nm. The grid used in (a), (j) and (k)are carbon film on copper, and that used in the other figures is laceycarbon film on nickel.

FIG. 5: (a) XPS survey spectra of g-C₃N₄ and BP/g-C₃N₄ nanosheets.High-resolution (b) C1 s, (c) Nis, and (d) P2p XPS spectra of BP/g-C₃N₄sample.

FIG. 6: (a) XRD patterns of bulk BP, BP nanosheets, g-C₃N₄ and BP/g-C₃N₄samples. (b) Amplification of XRD patterns of bulk BP and BP nanosheetsin the low-angle range which is marked by the dashed rectangle in (a).(c) UV-vis-NIR absorption spectra of BP nanosheets in IPA, and g-C₃N₄and BP/g-C₃N₄ powder samples. Insets in (c) are the photos of g-C₃N₄(bottom) and BP/g-C₃N₄ (top) powders.

FIG. 7: (a) Photocatalytic water splitting for H₂ evolution and (b) H₂evolution rate by BP (orange), g-C₃N₄ (blue) and BP/g-C₃N₄ (red)photocatalysts under visible light irradiation (λ>420 nm). (c) EISNyquist plots of g-C₃N₄ and BP/g-C₃N₄ with and without illumination. (d)Transient photocurrent density response of g-C₃N₄ and BP/g-C₃N₄ duringlight on/off cycles under a 0.2 V bias versus Ag/AgCl electrode.

FIG. 8: P2p XPS spectra of BP and BP/g-C₃N₄ samples after watersplitting under visible light irradiation for 24 hours.

FIG. 9: Valence band UPS cut-off spectra of (a) BP and (b) g-C₃N₄samples. (c) Schematic energy diagram of BP/g-C₃N₄ photocatalyst andproposed possible mechanism for the photocatalytic H₂ evolution fromwater splitting under visible light irradiation.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Before the present invention is further described, it is to beunderstood that the invention is not limited to the particularembodiments described below, as variations of these embodiments may bemade and still fall within the scope of the appended claims. It is alsoto be understood that the terminology employed is for the purpose ofdescribing particular embodiments, and is not intended to be limiting.Instead, the scope of the present invention will be established by theappended claims.

In order to provide a clear and consistent understanding of the termsused in the present specification, a number of definitions are providedbelow. Moreover, unless defined otherwise, all technical and scientificterms as used herein have the same meaning as commonly understood to oneof ordinary skill in the art to which this disclosure pertains.

As used herein, the term “exfoliation” refers to a process which allowsfor the separation of layers of a layer-structured material. The processmay involve dispersing the material into a solvent. The process isherein referred to as “ice-assisted exfoliation” or “solventice-assisted exfoliation”. The expressions “ice-assisted exfoliation”and “solvent ice-assisted exfoliation” are used herein interchangeably.

As used herein the expression “few-layer black phosphorous (BP)nanomaterial” is used interchangeably with the expression “few-layerblack phosphorous (BP) nanosheets” to refer to the material prepared bythe method according to the invention. As will be understood by askilled person, the “few-layer black phosphorous (BP) nanomaterial”according to the invention comprises nanosheets.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims and/or the specification may mean “one”, butit is also consistent with the meaning of “one or more”, “at least one”,and “one or more than one”. Similarly, the word “another” may mean atleast a second or more.

As used in this specification and claim(s), the words “comprising” (andany form of comprising, such as “comprise” and “comprises”), “having”(and any form of having, such as “have” and “has”), “including” (and anyform of including, such as “include” and “includes”) or “containing”(and any form of containing, such as “contain” and “contains”), areinclusive or open-ended and do not exclude additional, unrecitedelements or process steps.

As used herein when referring to numerical values or percentages, theterm “about” includes variations due to the methods used to determinethe values or percentages, statistical variance and human error.Moreover, each numerical parameter in this application should at leastbe construed in light of the number of reported significant digits andby applying ordinary rounding techniques.

The inventors have designed and performed a method for preparing ametal-free few-layer phosphorous nanomaterial. The method comprises anice-assisted exfoliation process (or solvent ice-assisted exfoliationprocess). The method according to the invention is novel, and allows forthe preparation of a few-layer phosphorous nanomaterial with improvedyield and reduced duration and exfoliation power.

In embodiments of the invention, the inventors have designed andperformed a method for preparing a metal-free few-layer blackphosphorous (BP) nanomaterial. In these embodiments, the ice-assistedexfoliation process involves use of a solvent. Preferably, the solventis an organic solvent, for example N-methyl-2-pyrrolidone (NMP).

In other embodiments of the invention, a photocatalyst is prepared. Inthese embodiments, the few-layer BP nanomaterial and graphitic carbonnitride (g-C₃N₄) are integrated into a single, 2D-on-2D architecture(BP/g-C₃N₄). The thus-obtained metal-free BP/g-C₃N₄ photocatalystexhibits a long-term stability, high photocatalytic H₂ evolutionefficiency from water, and good stability under visible lightirradiation.

The present invention is illustrated in further details by the followingnon-limiting examples.

EXPERIMENTAL SECTION

Materials. BP crystals of high-purity (99.998%) were purchased fromSmart Elements, N-methyl-2-pyrrolidone (NMP, 99.5%, anhydrous),isopropanol (IPA, 99.5%, anhydrous), urea (NH₂CONH₂), nitric acid(HNO₃), N,N-dimethylformamide (DMF) and triethanolamine (99.0%) werepurchased from Sigma-Aldrich and used as received without furtherpurification. The ultrapure water (18.2 MO cm, 25° C.), obtained from aMillipore Ultrapure water system, was used throughout the current study.

Example 1—Ice-Assisted Preparation of BP Nanosheets. BP nanosheets weresynthesized by developing a “NMP ice”-assisted exfoliation method.Specifically, 25 mg of bulk BP was ground into fine powder and dispersedinto 25 mL of NMP solvent. The dispersion was completely frozen with aliquid nitrogen bath for 5-10 minutes, and then sonicated in a bathsonicator (BRANSONIC, 70 W, 40 kHz) for about 10 minutes to make the“ice” melt. The procedure of freezing and melting was repeated 3 times.To protect the BP from oxygen and water, the dispersion was sealed in avial, and all the experimental manipulations were performed in aglovebox or with nitrogen bubbling. Afterwards, the dispersion wascentrifuged at 7000 rpm for 15 minutes to remove the residualun-exfoliated BP. The light yellow supernatant was decanted gently,which was the dispersion of BP nanosheets in NMP. The obtained BPnanosheets were washed with IPA by centrifugation at 12000 rpm, 2 times.The collected precipitate was re-dispersed into 25 mL of IPA. Theconcentration of BP in this dispersion was determined to be 0.75 mg mL⁻¹by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES).

Example 2—Preparation of g-C₃N₄ Nanosheets. The g-C₃N₄ nanosheets weresynthesized by our reported thermal polymerization method [12].Generally, urea (30 g) was placed into a covered alumina crucible andthen heated in a quartz tube furnace with a heating rate of 2° C. min⁻¹to 250, 350, and 550° C., and maintained at these three targettemperatures for 1, 2, and 2 hours, respectively. After being naturallycooled down to room temperature, the yellow powder was collected andwashed, three times with HNO₃ (0.1 mol L⁻¹) and water to removepotential alkaline residue (e.g., ammonia). After centrifugation, theprecipitate was dried in the vacuum at 80° C. overnight.

Example 3—Preparation of BP/g-C₃N₄ Photocatalysts. BP/g-C₃N₄ nanosheetswere prepared by dispersing 10 mg of g-C₃N₄ powder into 0.5 mL of BPnanosheets dispersion in IPA. The mixture was stirred for 2 hours tocouple BP nanosheets with g-C₃N₄ nanosheets under the protection of N₂.Subsequently, the sample was collected by centrifugation at 6000 rpm for5 minutes, and then washed completely with isopropanol. The finalproduct was obtained by drying the washed sample in an oven under vacuumat 60° C. overnight.

Example 4—Characterization. A transmission electron microscope (TEM,JEOL 2100F), equipped with an energy-dispersive X-ray (EDX)spectrometer, was employed and operated at an accelerating voltage of200 kV to study the microstructure and composition of the preparedsamples. The topography image of the BP nanosheets on the pre-cleanedglass was observed by an atomic force microscopy (AFM, Bruker, MultiMode8) in a tapping mode. Zeta potential of the as-prepared BP and g-C₃N₄nanosheets in IPA was recorded with a Brookhaven ZetaPlus system in astandard 10 mm all-side-transparent polymethyl methacrylate cuvette. Thecrystalline structure was analyzed by an X-ray diffraction system (XRD,PANalytical X′Pert MRD, operated at 45 kV and 40 mA) with a Cu Kαradiation source (A=0.15406 nm). X-ray photoelectron spectroscopy (XPS)was taken on a VG Escalab 220i-XL spectrometer equipped with a twinanode X-ray source. All the XPS spectra were calibrated with the C1speak at 284.8 eV as reference. Ultraviolet photoelectron spectroscopy(UPS) measurements were carried out with an unfiltered Helium (21.22 eV)gas discharge lamp to determine the valence band (VB) position of theas-prepared BP and g-C₃N₄ samples. The UV-visible-near infrared(UV-vis-NIR) absorption spectra of the BP nanosheets dispersion andBP/g-C₃N₄ powder were obtained using a scan spectrometer (Varian Cary5000). The concentration of BP nanosheets in IPA dispersion and thecontent of P in the composite samples were determined by an IRISIntrepid II XSP ICP-AES (Thermal Scientific, USA).

Example 5—Photoelectrochemical Measurements. Photoelectrochemical (PEC)properties were measured with a standard three electrode system in anelectrochemical workstation (CHI 660E, CH Instruments). The workingelectrode was prepared by coating the as-synthesized sample onfluorine-doped tin oxide (FTO) glass with its boundaries being protectedby Scotch tape. Specifically, 2 mg of powder sample was dispersed into 2mL of DMF under sonication for 30 minutes to obtain evenly dispersedslurry, which was drop-casted onto the FTO glass. After drying underambient condition, the epoxy resin glue was used to isolate the uncoatedpart of the FTO glass. A Pt wire and a Ag/AgCl electrode were used asthe counter and reference electrode, respectively. The 0.2 M of Na₂SO₄(pH=6.8) aqueous solution pre-purged with nitrogen for 30 minutes wasused as an electrolyte. A solar simulator equipped with an AM1.5G filter(LCS-100, Newport) was utilized as the light source. Nyquist plots wererecorded over the frequency range of 100 mHz to 100 kHz at a bias of 0.2V.

Example 6—Photocatalytic H₂ Evolution. Photocatalytic H₂ evolutionexperiment was performed in a 500 mL Pyrex top-irradiation reactor witha quartz cover. A 300 W Xenon lamp equipped with a cut-off filter (420nm) was used to provide the irradiation source in the visible wavelengthrange. Typically, 10 mg of photocatalysts were dispersed in 100 mL ofaqueous solution containing 10% of triethanolamine (TEOA) as sacrificialreagents. The mixture was deaerated by N₂ gas for 20 minutes andsonicated for 5 minutes. The system was sealed and vacuumed prior tophotocatalysis. During the irradiation, the suspension was stirredcontinuously and kept at a constant temperature by circulating coolingwater. The evolved H₂ was analyzed by a gas chromatography (GC, 7890B,Agilent Technologies) equipped with a thermal conductivity detector. Forstability measurements, the photocatalysts were collected from the finalreaction slurry by centrifugation, and then washed with ethanol andwater thoroughly. Subsequently, the recycled sample underwent thephotocatalytic H₂ evolution experiment under the identical conditionsand repeated for 5 cycles with a total irradiation time of 120 hours.

Results and Discussions

Preparation of BP Nanosheets and BP/g-C₃N₄ Photocatalysts

To prepare BP nanosheets, bulk BP crystals are exfoliated in NMP usingice-assisted ultrasonication as outlined above in Example 3 above, andschematically illustrated in FIG. 1a . When the bulk BP powder isdispersed into NMP, the spaces between BP layers are filled with thissolvent. As the melting point of NMP is −24° C., after being placed intodirect contact with liquid nitrogen bath, the dispersion starts tofreeze. The gradual growth of NMP ice crystals intercalates into BPlayers to enlarge the interlayer spacing of BP, which reduces theinterlayer Van der Waals interactions and will be favourable for theexfoliation process to generate BP nanosheets.

Subsequently, the frozen dispersion undergoes ultrasonication, and theBP nanosheets are exfoliated from the bulk BP. The ultrasonic vibrationof NMP ice between the layers also facilitates the exfoliation process.The required total time is less than 2 hours and the output power of thesonicator is less than 70 W. Compared with the conventional liquid phaseexfoliation [8a, 8c, 10], both the processing time and the sonicationpower are reduced in the method according to the invention. As a result,the BP nanosheets obtained is a good quality, with larger lateral sizeand less anomalous structural defects are obtained [8a, 11].Furthermore, the few-layer BP nanosheets are obtained in good yield.According to the ICP-AES analysis, 18.75 mg of few-layer BP nanosheetswere obtained from 25 mg of bulk BP with the yield of 75%, which ishigher than the values reported in the art; see Table 1 below. Theobtained BP nanosheets dispersion in IPA is brown and is stable. Indeed,no aggregation or color change is observed during storage for over fourweeks (FIGS. 2a-2b ).

TABLE 1 Few-layer BP nanosheets yield with different exfoliationmethods. Few- Sonication Bath Tip Sonicator layer Power Time Power TimeBP Reference (W) (h) (W) (h) yield ACS Nano, 2015, 9, 8869 70 13 26%Adv. Mater. 2016, 28, 380  20 30% 510 ACS Catal. 2016, 6, 8009 — 8 15%J. Am. Chem. Soc. 2017, 10 4 20% 139, 13234 Angew. Chem. Int. Ed. 10 420% 2018, 57, 1 The invention 70 2 75%

To form the 2D-on-2D assembly, the g-C₃N₄ powder was introduced into theBP dispersion (FIG. 2c ). The large amount of precipitate was soonobserved at the bottom of the solution with the supernatant turning tocolorless and transparent after the incubation at room temperature for30 minutes (FIG. 2d ), suggesting the successful integration andcoupling of BP nanosheets with g-C₃N₄ nanosheets. FIG. 2e presents thezeta potentials of BP and g-C₃N₄ in IPA, which are positive andnegative, respectively. A strong electrostatic interaction between themis noted. This contributes to their integration.

Morphological and Structural Characterization

The morphologies of the as-prepared BP nanosheets were characterized byTEM (FIG. 1b-1h ). The typical TEM image of BP nanosheets shows alamellar morphology with the lateral size of 50 nm-3 μm (FIG. 1b andFIGS. 3a-3d ). Only the peaks of C, Cu and P elements were observed inthe EDX spectrum (FIG. 1c ), indicating that the pure BP withoutoxidation was obtained via the ice-assisted exfoliation method. The BPnanosheets thickness distribution was investigated using AFM heightmeasurements (FIGS. 1d-f ). Lines 1 and 2 in FIG. 1d are randomlyselected and their corresponding height profiles are displayed in FIG.1e . Assuming the thickness of monolayer BP is 0.53 nm [6a, 6b, 8d], thenumber of layers of the generated BP nanosheets could be estimated fromthe AFM height measurements. FIG. 1f shows the statistical histogram ofthe number of BP layer distribution, which was obtained from the heightprofiles of 150 randomly selected individual BP nanosheets in AFMimages. The mean number of layers was determined to be <N>=5.9±1.5, andabout 93% of the observed BP nanosheets have the thickness of less than10 nm.

The g-C₃N₄ shows a free-standing graphene-like wrinkled nanosheetstructure (FIG. 4a ). As displayed in FIGS. 4b-4d , the initialmorphologies of BP and g-C₃N₄ nanosheets were not altered by theirintegration. The nanosheets marked with arrows in FIG. 4d are supposedto be BP considering their relatively regular edges, which are furthercorroborated by the high-angle annular dark field (HAADF) scanning TEM(STEM) image (FIG. 4e ) and its corresponding STEM-EDX elementalmappings (FIGS. 4f-4i ). The STEM-EDX mapping of C, N and P clearlyconfirms the co-existence of g-C₃N₄ and BP, and evidently shows thestacking of these two components. The high-resolution TEM (HRTEM) imagereveals lattice fringes of 0.34 nm and 0.26 nm, attributed to the (021)and (040) planes of the BP crystals (FIG. 4j ) [6e]. The presence of C,N and P peaks indicates the successful preparation of BP/g-C₃N₄ hybridnanosheets with high purity and without detectable oxidative degradation(FIG. 4g ), which is consistent with the STEM-EDX mapping results and isfurther verified by the following XPS analysis.

The composition and the chemical states of the as-prepared samples areassessed using XPS (FIG. 5). In the XPS survey spectra of BP/g-C₃N₄(FIG. 5a ), only the peaks assigned to C, N, O and P elements wereobserved, signifying the high purity of the prepared samples and thesuccessful integration of BP and g-C₃N₄ nanosheets. As outlined above,01s peak was observed in the XPS spectrum of g-C₃N₄, which is attributedto the 0 element in the adsorbed O₂ or H₂O on the sample surface [13].The similar atomic 0 percentages of g-C₃N₄ (3.61%) and BP/g-C₃N₄ (3.59%)illustrates that no further oxidation occurred in the preparation ofBP/g-C₃N₄ hybrid sample; see Table 2 below. In addition, theconcentration of BP in BP/g-C₃N₄ nanosheets was detected to be 3.3% byXPS, which is quite close to that of 3.61% measured by ICP-AES and thenominal value of 3.75%.

TABLE 2 Atomic composition of g-C₃N₄ and BP/g-C₃N₄ photocatalysts. Catom N atom O atom P atom Sample (%) (%) (%) (%) g-C₃N₄ 46.71 49.68 3.610 BP/g-C₃N₄ 46.70 47.41 3.59 3.30

These results suggest the effective coupling between BP and g-C₃N₄nanosheets. To specify the bond formation in the prepared BP/g-C₃N₄sample, peak deconvolution was performed for the C1 s, N1s and P2p XPSspectra (FIGS. 5b-5d ). The high-resolution C1s XPS spectrum presentstwo distinct peaks at 284.8 and 288.3 eV (FIG. 5b ), which can beassigned to the graphitic sp² C═C bonds in the surface adventitiouscarbonaceous environment and in the C—N aromatic heterocycles,respectively [4c, 14]. The main N1s peak was deconvoluted into threepeaks (FIG. 5c ), located at 398.6, 399.4 and 401.1 eV, which areassigned to the sp² hybridized N in triazine rings (C═N—C), tertiary N(N—(C)₃) and amino group (C—N—H), respectively [15]. As shown in FIG. 5d, the fitting result of P2p spectrum shows two peaks at binding energiesof 129.8 and 130.9 eV, corresponding to P2p_(3/2) and P2p_(1/2),respectively. It is worth noting that the peak in the range of133.5-134.0 eV, originating from oxidized P (P_(x)O_(y)) [7c, 7d, 16],was not observed in the P2p XPS spectrum, indicating that P was notoxidized during both the exfoliation of bulk BP to BP nanosheets and thepreparation of BP/g-C₃N₄ hybrid sample. The time-efficient ice-assistedexfoliation method according to the invention plays an important role inprotecting BP from oxidation by largely shortening the ultrasonicationtime and further reducing the possibility of exposure to O₂.

FIG. 6 shows the XRD patterns of bulk BP, exfoliated BP nanosheets,g-C₃N₄ and BP/g-C₃N₄ samples. As illustrated in FIG. 6a , thediffraction peaks shown in the patterns of bulk BP and BP nanosheets canbe indexed to the orthorhombic BP with space group Cmca (64) accordingto the standard pattern of BP (JCPDS No. 73-1358) [6d, 6f]. Furthermore,the low-angle peak originated from the periodic stacking of layersexhibits a downshift from 16.95° of the BP bulk counterpart to 15.89° ofthe exfoliated BP nanosheets, corresponding to the inter-plane distanceincreasing from the 5.2 Å to 5.6 Å, respectively (FIG. 6b ). This resultshows that intercalation of ice crystals can enlarge the inter-planarspacing of BP, and further benefit its exfoliation by reducing theinterlayer Van der Waals interactions. In the XRD pattern of g-C₃N₄, thetwo peaks at 13.0° and 27.4° are ascribed to the in-planar arrangementof the tri-s-triazine unit and the inter-planar stacking of theconjugated aromatic system, respectively [2a, 4c, 12a, 15c, 17]. For thediffractogram of BP/g-C₃N₄ sample, both the characteristic diffractionpeaks of BP and g-C₃N₄ were observed, explicitly confirming theirsuccessful integration once again.

The optical properties of BP nanosheets in IPA, g-C₃N₄ and BP/g-C₃N₄nanosheets were investigated as displayed in the UV-vis-NIR absorptionspectra (FIG. 6c ). The BP nanosheets show a quite broad absorption bandfrom UV to NIR regions with the absorption edge of 910 nm, correspondingto its bandgap of about 1.36 eV. The g-C₃N₄ exhibits a typicalsemiconductor-like absorption spectrum in the UV and blue regions withthe absorption edge of around 459 nm, representing the bandgap of about2.70 eV [2a, 12a]. For the BP/g-C₃N₄ 2 D-on-2D assembled nanosheetphotocatalyst, in addition to the absorption of g-C₃N₄, an enhanced tailabsorption in the visible and NIR regions was observed due to theintroduction of BP nanosheets. This can be propitious to the visiblelight-driven photocatalytic water splitting for H₂ production.

Photocatalytic H₂ Evolution

The photocatalytic H₂ production from water splitting by BP, g-C₃N₄ andBP/g-C₃N₄ photocatalysts under visible light irradiation and thestability measurement of BP/g-C₃N₄ are shown in FIGS. 7a-7b . All thesamples show H₂ evolution from water containing triethanolamine, whichacts as the sacrificial electron donor to quench the photoinduced holesunder visible light irradiation (λ>420 nm). The as-prepared BP/g-C₃N₄photocatalyst exhibits much larger H₂ evolution amount (93.14 μmol),compared to that of BP (13.18 μmol) and g-C₃N₄ samples (20.43 μmol)after 24 hours of light irradiation.

As displayed in FIG. 7b , the highest H₂ evolution rate was achieved byBP/g-C₃N₄ (384.17 μmol g⁻¹ h⁻¹), which is about 7 times and 4.5 timeshigher than that of pure BP (54.88 μmol g⁻¹ h⁻¹) and g-C₃N₄ (86.23 μmolg⁻¹ h⁻¹). The fast recombination of photo-generated charge carriers inBP and g-C₃N₄ is probably responsible for their poorer activity. Theintegration of g-C₃N₄ and BP nanosheets improved the visible lightphotocatalytic activity in water splitting. The excited electrons inconduction band (CB) of g-C₃N₄ can be transferred to BP nanosheets andsuppress the recombination of charge carriers in g-C₃N₄, and furtherenhance the photocatalytic activity. The H₂ production rate obtained byBP/g-C₃N₄ is comparable to or higher than that of the photocatalyst withthe loading of precious metal as co-catalyst reported in the art; seeTable 3 below.

TABLE 3 Photocatalytic H₂ production rate under visible light (λ > 420nm) irradiation. H₂ evolution rate (μmol g⁻¹ References Metal Catalystsh⁻¹) Nat. Mater. 2009, 8, 3 wt % Pt C₃N₄ 106.94 76 Chem. Mater. 2015, 1wt % Pt H₂ treated g-C₃N₄ 29.63 27, 4930 J. Catal. 2016, 342, 55 1 wt %Pt g-C₃N₄ anatase/ 29.97 brookite TiO₂ Appl. Catal., B 2016, 3 wt % PtBr-modified g-C₃N₄ 960 196, 112 Adv. Mater. 2017, 3 wt % Pt crystallineCN 1060 1700008 nanosheets Appl. Catal., B 2018, 3 wt % Pt O-doped C₃N₄732 224, 1 nanorods Science 2015, 347, free CDots-C₃N₄ 105 970 Angew.Chem. Int. Ed. free BP/BiVO₄ 160 2018, 57, 6 The invention freeBP/g-C₃N₄ 384.17

Furthermore, only about 2% decrease was observed in the H₂ evolution bythe as-synthesized BP/g-C₃N₄ photocatalyst after 120 hours of visiblelight irradiation, suggesting that it possesses good stability in waterunder light illumination. The XPS spectra of BP and BP/g-C₃N₄ afterphotocatalytic experiment were measured (FIG. 8). One additional peak atabout 134 eV, assigned to the oxidized P, was observed in their P2p XPSspectra compared to the spectra before water splitting, which accountsfor 21.64% and 7.56% in the three peaks of BP and BP/g-C₃N₄,respectively; see Table 4 below, indicating that the introduction ofg-C₃N₄ inhibits the oxidation of BP. Though the P in BP/g-C₃N₄ wasslightly oxidized, the photocatalytic activity was not distinctivelyaffected. These results suggest that the as-prepared BP/g-C₃N₄ is aneconomic, efficient and stable, metal-free photocatalyst, withoutintroducing any metal as co-catalyst, for H₂ evolution from watersplitting under visible light.

TABLE 4 The atomic composition of P1, P2, and P3 of BP and BP/g-C₃N₄photocatalysts in FIG. 8. P1 P2 P3 Sample (%) (%) (%) BP 21.64 21.1857.19 BP/g-C₃N₄ 7.56 39.86 52.57

PEC Measurements

The PEC properties of the as-prepared g-C₃N₄ and BP/g-C₃N₄ samples wereevaluated by electrochemical impedance spectroscopy (EIS) and transientphotocurrent responses (FIGS. 7c-7d ). Some useful information for thecharge transfer resistance can be shown in the high frequency region ofNyquist plots. The decreased arc radii were exhibited in the EIS Nyquistplots of BP/g-C₃N₄ compared to that of g-C₃N₄ both in the dark and undersimulated solar light irradiation (FIG. 7c ), suggesting that theintroduction of BP leads to enhanced electronic conductivity and thusincrease the interfacial charge transfer rate in BP/g-C₃N₄ sample [2d,12a, 15c, 18].

To further verify the charge separation transfer performance, thetransient photocurrent responses for more than ten light on-off cycleswere measured under simulated solar light irradiation (FIG. 7d ). Thephotocurrent density rapidly increases to a saturation value and remainsconstant once the light is switched on, and immediately returns tonearly zero when the light is turned off. The saturated photocurrentdensity of BP/g-C₃N₄ (about 5.28 μA cm⁻²) is about 4.8 times higher thanthat of plain g-C₃N₄ photocatalysts (about 1.11 μA cm⁻²). The increasedphotocurrent density shows that the introduction of BP nanosheets canincrease the mobility, facilitate the separation or elongate the lifetime of the photo-generated charge carriers [2d, 19], and/or enhance thevisible light absorption due to the narrower bandgap. Altogether theycontribute to the improved photocatalytic H₂ evolution rate of watersplitting under visible light irradiation. It is worth noting thatalmost no decrease in the photocurrent density was observed after about2000 s of the light on-off tests, which shows that the as-synthesizedg-C₃N₄ and BP/g-C₃N₄ samples possesses good stability under lightirradiation.

UPS Measurement and Mechanism of Photocatalytic H₂ Evolution

To better understand the nature of BP/g-C₃N₄ as an efficientphotocatalyst for H₂ evolution, UPS measurements were performed todetermine the energy levels of BP and g-C₃N₄ nanosheets (FIGS. 9a-9b ).The intersections of the extrapolated linear portion at high and lowbinding energies with the baseline give the edges of the UPS spectra,from which the UPS widths of BP and g-C₃N₄ are determined to be 15.99 eVand 14.95 eV, respectively [14a]. Then the VB energy (E_(VB)) values ofBP and g-C₃N₄ are calculated to be 5.23 eV and 5.96 eV, respectively, bysubtracting the width of the UPS spectra from the excitation energy(21.22 eV). Combining with the measured bandgap energy (E_(g)) from theabsorption spectra, the CB energy values (E_(CB)) of BP (3.87 eV) andg-C₃N₄ (3.26 eV) are estimated from E_(CB)=E_(VB)−E_(g) [7c, 14a]. Thesevalues in eV are all converted to electrochemical energy potentials in Vaccording to the reference standard for which −4.44 eV vs. vacuum levelequals 0 V vs. reversible hydrogen electrode (RHE) [14a], which are alldisplayed in FIG. 9 c.

Being based on the UPS measurement results, the possible mechanism forthe largely enhanced photocatalytic activity in H₂ evolution ofBP/g-C₃N₄ photocatalysts is proposed. As schematically illustrated inFIG. 9c , the CB energy level of BP is more negative than that ofg-C₃N₄, and both are more positive than the reduction potential ofH⁺/H₂. In addition, the VB energy level of BP is higher than that ofg-C₃N₄. These properly positioned bands are suitable for the transfer ofcharge carriers for water splitting, corroborating the capability ofBP/g-C₃N₄ as a metal-free photocatalyst for H₂ evolution. Under visiblelight irradiation, mainly the electrons in the VB of g-C₃N₄ are excitedto its CB, leaving behind the positive-charged holes in the VB.Afterwards, the excited electrons can be further transferred into the CBof adjacent BP, suppressing the recombination of charge carriers andpromote the reduction of H₂O to produce H₂. At the same time, the holesin the VB of g-C₃N₄ can be immediately captured by the hole-sacrificialagent TEOA to generate its oxide. In this process, BP plays a role asthe electron sink to inhibit the charge carriers recombination and leadsto efficient H₂ evolution under visible light irradiation, which is inagreement with the PEC measurement.

As will be understood by a skilled person, other allotropes of BP may beused as starting materials. Such materials which generally present alayered structure include but are not limited to red phosphorous (RP)and violet phosphorous (VP).

As will be understood by a skilled person, other organic solvents may beused in the ice-assisted process. Such solvents are suitably selectedsuch as not to allow for any oxidation to occur. In particular, suchsolvents include but are not limited to alcohols such as methanol,ethanol and isopropanol (IPA), diethyl ether, chloroform,tetrahydrofuran, cyclohexane, toluene, dimethylformamide and the like,and combinations thereof, in addition to N-methyl-2-pyrrolidone (NMP).

As will be understood by a skilled person, other organic solvents may beused for the purification of the nanosheets formed, i.e., in the washingand re-dispersion steps. Such solvents are suitable selected to allowdispersion of the formed nanosheets. The solvent for these separationssteps may be the same or different. Such solvents are suitably selectedsuch as not to allow for any oxidation to occur. In particular, forexample the solvents for the washing step include but are not limited toother alcohols such as methanol, ethanol, in addition to isopropanol(IPA); diethyl ether, chloroform, tetrahydrofuran, cyclohexane, toluene,dimethylformamide, and the like, and combinations thereof. And thesolvents for the re-dispersion step include but are not limited to otheralcohols such as methanol, ethanol, in addition to isopropanol (IPA);diethyl ether, chloroform, tetrahydrofuran, cyclohexane, toluene,dimethylformamide, N-methyl-2-pyrrolidone (NMP), and the like, andcombinations thereof. Accordingly, as will be understood by a skilledperson, the solvent used in the purification step (washing and/orre-dispersion steps) may be the same as the solvent used in theice-assisted process.

As will be understood by a skilled person, any suitable 2D material maybe coupled with the few-layer phosphorous nanomaterial according to theinvention such as to obtain a photocatalyst. Such material may be poly(methyl methacrylate), graphene or hexagonal boron nitride which may benitrogen-doped, molybdenum disulfide, a carbon nitride nanomaterial, andthe like, in addition to graphitic carbon nitride (g-C₃N₄).

The scope of the claims should not be limited by the preferredembodiments set forth in the examples, but should be given the broadestinterpretation consistent with the description as a whole.

The present description refers to a number of documents, the content ofwhich is herein incorporated by reference in their entirety.

REFERENCES

-   [1] a) A. Fujishima, K. Honda, Nature 1972, 238, 37-38; b) K.    Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue, K.    Domen, Nature 2006, 440, 295-295; c) A. Kudo, Y. Miseki, Chem. Soc.    Rev. 2009, 38, 253-278; d) Q. Zhang, D. Thrithamarassery    Gangadharan, Y. Liu, Z. Xu, M. Chaker, D. Ma, J. Materiomics 2017,    3, 33-50.-   [2] a) X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M.    Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8,    76-80; b) G. G. Zhang, Z. A. Lan, X. C. Wang, Angew. Chem. Int. Ed.    2016, 55, 15712-15727; c) H. H. Ou, P. J. Yang, L. H. Lin, M.    Anpo, X. C. Wang, Angew. Chem. Int. Ed. 2017, 56, 10905-10910; d) D.    Zheng, X. N. Cao, X. Wang, Angew. Chem. Int. Ed. 2016, 55,    11512-11516.-   [3] J. S. Zhang, X. F. Chen, K. Takanabe, K. Maeda, K. Domen, J. D.    Epping, X. Z. Fu, M. Antonietti, X. C. Wang, Angew. Chem. Int. Ed.    2010, 49, 441-444.-   [4] a) X. F. Chen, J. S. Zhang, X. Z. Fu, M. Antonietti, X. C.    Wang, J. Am. Chem. Soc. 2009, 131, 11658-11659; b) L. Sun, M. J.    Yang, J. F. Huang, D. S. Yu, W. Hong, X. D. Chen, Adv. Funct. Mater.    2016, 26, 4943-4950; c) V. W.-h. Lau, I. Moudrakovski, T. Botari, S.    Weinberger, M. B. Mesch, V. Duppel, J. Senker, V. Blum, B. V.    Lotsch, Nat. Commun. 2016, 7; d) W. J. Ong, L. L. Tan, Y. H.    Ng, S. T. Yong, S. P. Chai, Chem. Rev. 2016, 116, 7159-7329; e) J.    Fu, J. Yu, C. Jiang, B. Cheng, Adv. Energy Mater. 2017, 1701503.-   [5] a) Y. X. Deng, Z. Luo, N. J. Conrad, H. Liu, Y. J. Gong, S.    Najmaei, P. M. Ajayan, J. Lou, X. F. Xu, P. D. Ye, ACS Nano 2014, 8,    8292-8299; b) L. Li, Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D.    Feng, X. H. Chen, Y. Zhang, Nat. Nanotechnol. 2014, 9,    372-377; c) H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. F. Xu, D.    Tomanek, P. D. Ye, ACS Nano 2014, 8, 4033-4041; d) E. S. Reich,    Nature 2014, 506, 19; e) F. N. Xia, H. Wang, D. Xiao, M. Dubey, A.    Ramasubramaniam, Nat. Photonics 2014, 8, 899-907; f) X. Ling, H.    Wang, S. X. Huang, F. N. Xia, M. S. Dresselhaus, PNAS 2015, 112,    4523-4530; g) L. Z. Kou, C. F. Chen, S. C. Smith, J. Phys. Chem.    Lett. 2015, 6, 2794-2805; h) H. Liu, Y. C. Du, Y. X. Deng, P. D. Ye,    Chem. Soc. Rev. 2015, 44, 2732-2743; i) C. R. Ryder, J. D.    Wood, S. A. Wells, Y. Yang, D. Jariwala, T. J. Marks, G. C.    Schatz, M. C. Hersam, Nat. Chem. 2016, 8, 598-603.-   [6] a) F. N. Xia, H. Wang, Y. C. Jia, Nat. Commun. 2014, 5,    4458; b) M. Buscema, D. J. Groenendijk, S. I. Blanter, G. A.    Steele, H. S. J. van der Zant, A. Castellanos-Gomez, Nano Lett.    2014, 14, 3347-3352; c) J. Sun, G. Y. Zheng, H. W. Lee, N.    Liu, H. T. Wang, H. B. Yao, W. S. Yang, Y. Cui, Nano Lett. 2014, 14,    4573-4580; d) H. Wang, X. Z. Yang, W. Shao, S. C. Chen, J. F.    Xie, X. D. Zhang, J. Wang, Y. Xie, J. Am. Chem. Soc. 2015, 137,    11376-11382; e) Z. Sun, H. Xie, S. Tang, X. F. Yu, Z. Guo, J.    Shao, H. Zhang, H. Huang, H. Wang, P. K. Chu, Angew. Chem. Int. Ed.    2015, 54, 11526-11530; f) X. Zhang, H. Xie, Z. Liu, C. Tan, Z.    Luo, H. Li, J. Lin, L. Sun, W. Chen, Z. Xu, L. Xie, W. Huang, H.    Zhang, Angew. Chem. Int. Ed. 2015, 54, 3653-3657; g) Y. Yang, J.    Gao, Z. Zhang, S. Xiao, H. H. Xie, Z. B. Sun, J. H. Wang, C. H.    Zhou, Y. W. Wang, X. Y. Guo, P. K. Chu, X. F. Yu, Adv. Mater. 2016,    28, 8937-8944.-   [7] a) M. Z. Rahman, C. W. Kwong, K. Davey, S. Z. Qiao, Energy    Environ. Sci. 2016, 9, 709-728; b) W. Y. Lei, T. T. Zhang, P.    Liu, J. A. Rodriguez, G. Liu, M. H. Liu, ACS Catal. 2016, 6,    8009-8020; c) M. Zhu, S. Kim, L. Mao, M. Fujitsuka, J. Zhang, X.    Wang, T. Majima, J. Am. Chem. Soc. 2017, 139, 13234-13242; d) M. S.    Zhu, X. Y. Cai, M. Fujitsuka, J. Y. Zhang, T. Majima, Angew. Chem.    Int. Ed. 2017, 56, 2064-2068; e) X. J. Zhu, T. M. Zhang, Z. J.    Sun, H. L. Chen, J. Guan, X. Chen, H. X. Ji, P. W. Du, S. F. Yang,    Adv. Mater. 2017, 29; f) W. Hu, L. Lin, R. Zhang, C. Yang, J.    Yang, J. Am. Chem. Soc. 2017, 139, 15429-15436.-   [8] a) A. H. Woomer, T. W. Farnsworth, J. Hu, R. A. Wells, C. L.    Donley, S. C. Warren, ACS Nano 2015, 9, 8869-8884; b) A. Ziletti, A.    Carvalho, D. K. Campbell, D. F. Coker, A. H. C. Neto, Phys. Rev.    Lett. 2015, 114, 046801; c) J. Kang, J. D. Wood, S. A. Wells, J. H.    Lee, X. L. Liu, K. S. Chen, M. C. Hersam, ACS Nano 2015, 9,    3596-3604; d) A. Favron, E. Gaufres, F. Fossard, A. L.    Phaneuf-L'Heureux, N. Y. W. Tang, P. L. Levesque, A. Loiseau, R.    Leonelli, S. Francoeur, R. Martel, Nat. Mater. 2015, 14,    826-832; e) A. Hirsch, F. Hauke, Angew. Chem. Int. Ed. 2017, 57,    4338-4354.-   [9] a) J. D. Wood, S. A. Wells, D. Jariwala, K. S. Chen, E.    Cho, V. K. Sangwan, X. L. Liu, L. J. Lauhon, T. J. Marks, M. C.    Hersam, Nano Lett. 2014, 14, 6964-6970; b) R. A. Doganov, E. C. T.    O'Farrell, S. P. Koenig, Y. T. Yeo, A. Ziletti, A. Carvalho, D. K.    Campbell, D. F. Coker, K. Watanabe, T. Taniguchi, A. H. C. Neto, B.    Ozyilmaz, Nat. Commun. 2015, 6; c) W. N. Zhu, M. N. Yogeesh, S. X.    Yang, S. H. Aldave, J. S. Kim, S. Sonde, L. Tao, N. S. Lu, D.    Akinwande, Nano Lett. 2015, 15, 1883-1890; d) Y. T. Zhao, H. Y.    Wang, H. Huang, Q. L. Xiao, Y. H. Xu, Z. N. Guo, H. H. Xie, J. D.    Shao, Z. B. Sun, W. J. Han, X. F. Yu, P. H. Li, P. K. Chu, Angew.    Chem. Int. Ed. 2016, 55, 5003-5007.-   [10] a) J. R. Brent, N. Savjani, E. A. Lewis, S. J. Haigh, D. J.    Lewis, P. O'Brien, Chem. Commun. 2014, 50, 13338-13341; b) P.    Yasaei, B. Kumar, T. Foroozan, C. H. Wang, M. Asadi, D.    Tuschel, J. E. Indacochea, R. F. Klie, A. Salehi-Khojin, Adv. Mater.    2015, 27, 1887-1892; c) L. Chen, G. M. Zhou, Z. B. Liu, X. M. Ma, J.    Chen, Z. Y. Zhang, X. L. Ma, F. Li, H. M. Cheng, W. C. Ren, Adv.    Mater. 2016, 28, 510-517.-   [11] M. Batmunkh, C. J. Shearer, M. J. Biggs, J. G. Shapter, J.    Mater. Chem. A 2016, 4, 2605-2616.-   [12] a) Q. Zhang, J. Deng, Z. Xu, M. Chaker, D. Ma, ACS Catal. 2017,    7, 6225-6234; b) Z. Xu, M. G. Kibria, B. AlOtaibi, P. N.    Duchesne, L. V. Besteiro, Y. Gao, Q. Zhang, Z. Mi, P. Zhang, A. O.    Govorov, L. Mai, M. Chaker, D. Ma, Appl. Catal., B 2018, 221, 77-85.-   [13] a) F. Dong, Z. W. Zhao, T. Xiong, Z. L. Ni, W. D. Zhang, Y. J.    Sun, W. K. Ho, ACS Appl. Mater. Interfaces 2013, 5,    11392-11401; b) Y. Q. Cao, Z. Z. Zhang, J. L. Long, J. Liang, H.    Lin, H. X. Lin, X. X. Wang, J. Mater. Chem. A 2014, 2, 17797-17807.-   [14] a) J. Liu, Y. Liu, N. Y. Liu, Y. Z. Han, X. Zhang, H. Huang, Y.    Lifshitz, S. T. Lee, J. Zhong, Z. H. Kang, Science 2015, 347,    970-974; b) H. J. Kong, D. H. Won, J. Kim, S. I. Woo, Chem. Mater.    2016, 28, 1318-1324.-   [15] a) C. Ye, J.-X. Li, Z.-J. Li, X.-B. Li, X.-B. Fan, L.-P.    Zhang, B. Chen, C.-H. Tung, L.-Z. Wu, ACS Catal. 2015, 5,    6973-6979; b) J. Q. Zhang, X. H. An, N. Lin, W. T. Wu, L. Z.    Wang, Z. T. Li, R. Q. Wang, Y. Wang, J. X. Liu, M. B. Wu, Carbon    2016, 100, 450-455; c) G. Peng, L. Xing, J. Barrio, M. Volokh, M.    Shalom, Angew. Chem. Int. Ed. 2017, 56, 1-7; d) H. J. Yu, R.    Shi, Y. X. Zhao, T. Bian, Y. F. Zhao, C. Zhou, G. I. N.    Waterhouse, L. Z. Wu, C. H. Tung, T. R. Zhang, Adv. Mater. 2017, 29,    1605148.-   [16] M. Zhu, Z. Sun, M. Fujitsuka, T. Majima, Angew. Chem. Int. Ed.    2018, 57, 1-6.-   [17] a) D. J. Martin, P. J. T. Reardon, S. J. A. Moniz, J. W.    Tang, J. Am. Chem. Soc. 2014, 136, 12568-12571; b) Q. Han, B.    Wang, J. Gao, Z. Cheng, Y. Zhao, Z. Zhang, L. Qu, ACS Nano 2016, 10,    2745-2751.-   [18[ M. X. Li, W. J. Luo, D. P. Cao, X. Zhao, Z. S. Li, T. Yu, Z. G.    Zou, Angew. Chem. Int. Ed. 2013, 52, 11016-11020.-   [19] D. Shi, R. Zheng, M. J. Sun, X. Cao, C. X. Sun, C. J.    Cui, C. S. Liu, J. Zhao, M. Du, Angew. Chem. Int. Ed. 2017, 56,    14637-14641.

1. Method for preparing a few-layer phosphorous nanomaterial from a bulklayer-structured phosphorous material, comprising an ice-assistedexfoliation process or solvent ice-assisted exfoliation process. 2.Method for preparing a few-layer phosphorous nanomaterial from a bulklayer-structured phosphorous material, comprising a combination of thefollowing steps: grinding, dispersion in a solvent, freezing, melting,separation, purification.
 3. Method for preparing a few-layerphosphorous nanomaterial, comprising: (a) providing a bulklayer-structured phosphorous material; (b) grinding the bulk phosphorousmaterial; (c) dispersing the grinded material into a first solvent toobtain a first dispersion; (d) freezing the first dispersion for aperiod of time, preferably using liquid nitrogen; (e) melting the frozendispersion, preferably by sonication for a period of time to obtain asecond dispersion; and (f) submitting the second dispersion to aseparation step, preferably involving centrifugation for a period oftime, to obtain the nanomaterial.
 4. Method according to claim 3,further comprising a purification step; preferably the purification stepcomprises: (g) washing the nanomaterial using a second solvent,optionally repeating step (g) a number of time, preferably 2-6 times, or3 times, or 4 times; and (h) dispersing the nanomaterial into a thirdsolvent, wherein the second and third solvents are the same ordifferent.
 5. Method according to claim 3 or 4, wherein steps (d) and(e) are repeated a number of time, preferably 2 to 6 times, or 3 timesor 4 times.
 6. Method according to claim 3, wherein the freezing timeperiod at step (d) is about 3-15 minutes, or about 4-14 minutes, orabout 5-13 minutes, or about 5-12 minutes, or about 5-11 minutes, orabout 5-10 minutes, or about 6-8 minutes.
 7. Method according to claim3, wherein the sonication time period at step (e) is about 5-15 minutes,or about 6-14 minutes, or about 7-13 minutes, or about minutes 8-12minutes, or about 9-11 minutes, or about 10 minutes.
 8. Method accordingto claim 3, wherein the centrifugation at step (f) is performed at 7000rpm and the time period is about 10-20 minutes, or about 12-18 minutes,or about 14-16 minutes, or about 15 minutes.
 9. Method according to anyone of claims 1 to 8, wherein the bulk layered structure phosphorousmaterial is black phosphorous (BP), red phosphorous (RP), violetphosphorous (VP).
 10. Method according to any one of claims 1 to 9,wherein the bulk layer-structured phosphorous material is a blackphosphorous (BP) material, and the few-layer phosphorous nanomaterial isa few-layer black phosphorous (BP) nanomaterial.
 11. Method according toclaim 1, wherein the solvent is an organic solvent; preferably theorganic solvent is selected from the group consisting ofN-methyl-2-pyrrolidone (NMP), alcohols such as methanol, ethanol andisopropanol (IPA), diethyl ether, chloroform, tetrahydrofuran,cyclohexane, toluene, dimethylformamide, and combinations thereof; morepreferably the solvent is N-methyl-2-pyrrolidone (NMP).
 12. Methodaccording to claim 3 or 4, wherein: the first solvent is selected fromthe group consisting of N-methyl-2-pyrrolidone (NMP), alcohols such asmethanol, ethanol and isopropanol (IPA), diethyl ether, chloroform,tetrahydrofuran, cyclohexane, toluene, dimethylformamide, andcombinations thereof; preferably the first solvent isN-methyl-2-pyrrolidone (NMP); the second solvent is selected from thegroup consisting of isopropanol (IPA), other alcohols such as methanoland ethanol; diethyl ether, chloroform, tetrahydrofuran, cyclohexane,toluene, dimethylformamide, and combinations thereof; preferably thesecond solvent is isopropanol (IPA); and the third solvent is selectedfrom the group consisting of isopropanol (IPA), other alcohols such asmethanol and ethanol; diethyl ether, chloroform, tetrahydrofuran,cyclohexane, toluene, dimethylformamide, N-methyl-2-pyrrolidone (NMP),and combinations thereof; preferably the second solvent is isopropanol(IPA).
 13. Method according to any one claims 1 to 12, whereinsubstantially no oxidation occurs.
 14. Method according to any oneclaims 1 to 12, wherein the few-layer phosphorous nanomaterial ismetal-free.
 15. A few-layer phosphorous nanomaterial obtained by themethod as defined in any one of claims 1 to
 14. 16. A few-layer blackphosphorous (BP) nanomaterial obtained by the method as defined in anyone of claims 1 to
 14. 17. A few-layer phosphorous nanomaterial asdefined in claim 15 or 16, having 4 to 10 layers, or 5 to 9 layers, or 6to 8 layers, or 7 layers, or 6 layers.
 18. A few-layer phosphorousnanomaterial as defined in any one of claims 15 to 17, having athickness which is less than about 12 nm, or less than about 10 nm; orwhich is about 9 nm, or about 8 nm, or about 7 nm, or about 6 nm, orabout 5 nm.
 19. Use of a few-layer phosphorous nanomaterial as definedin any one of claims 15 to 18, in the development of photocatalysts,transistor devices, photodetector devices, solar cells, or inbio-imaging, or in phototherapy.
 20. A method for preparing aphotocatalyst, comprising coupling the few-layer phosphorousnanomaterial as defined in any one of claims 15 to 18, with a 2Dmaterial; preferably the 2D material is selected from the groupconsisting of poly (methyl methacrylate), graphene or hexagonal boronnitride which may be nitrogen-doped, molybdenum disulfide, a carbonnitride nanomaterial; more preferably the 2D material is graphiticcarbon nitride (g-C₃N₄).
 21. A method for preparing a photocatalyst,comprising coupling the few-layer black phosphorous (BP) nanomaterial asdefined in claim 20, with graphitic carbon nitride (g-C₃N₄).
 22. Use ofthe few-layer phosphorous nanomaterial as defined in any one of claims15 to 18, in the preparation of a photocatalyst.
 23. Use of thefew-layer black phosphorous (BP) nanomaterial as defined in claim 16, inthe preparation of a photocatalyst.
 24. A photocatalyst obtained by themethod as defined in claim 20 or
 21. 25. A photocatalyst obtained by themethod as defined in claim 21, which is few-layer black phosphorousnanomaterial/g-C₃N₄.
 26. Use of the photocatalyst as defined in claim 24or 25, for water splitting (H₂ evolution).